It wouldn't actually. The contact in his phone (incorrectly added by Apple AI from a forwarded email) would be the same regardless which app he was using.
Instead, Signal (and this forked version) would have to do its own independent contact management, maybe based on in-person scanning of QR codes plus web-of-trust.
If only it would a- not ask you to access your contacts and b- accept when you say no instead of saying "we'll ask again later" (and then, indeed, asking again later).
Instead, Signal (and this forked version) would have to do its own independent contact management, maybe based on in-person scanning of QR codes plus web-of-trust.